Typically, vehicles such as automobiles are occasionally installed with a traction control apparatus and the like to prevent drive slip. There has been known that, when an accelerator operation, a low-μ-road driving or the like causes drive slip, such a traction control apparatus performs a braking control of a brake and driving control of an engine to generate appropriate traction on wheels, thereby preventing wheel slip.
When the traction control apparatus is installed in a two-wheel-drive car, a vehicle speed can easily be estimated by detecting rotation speeds of driven wheels (not driving wheels) by a sensor and the like.
However, since all wheels of all-wheel-drive vehicles such as a four-wheel-car are driving wheels, all the wheels may generate drive slip. Accordingly, it is difficult to accurately estimate a vehicle speed only by detecting rotation speeds of all the wheels.
Accordingly, an all-wheel-drive vehicle is installed with a rotation speed sensor for wheels and an acceleration sensor. A select wheel to be referred to is selected based on a rotation speed of each of the wheels by the rotation speed sensor. A vehicle speed is estimated based on the rotation speed of the select wheel and an output from the acceleration sensor (see, for instance, Patent Literature 1).